Wellbores are drilled to locate and produce hydrocarbons. A downhole drilling tool with a bit at an end thereof is advanced into the ground to form a wellbore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The fluid exits the drill bit and flows back up to the surface for recirculation through the tool. The drilling mud may also be used to form a mud cake to line the wellbore. The drilling mud may be air, water or oil based drilling mud.
During the drilling operation, it may be desirable to perform various downhole evaluations, such as testing and/or sampling, (referred to herein as “logging”). In some cases, the drilling tool may be provided with devices to log the surrounding formation. In some cases, the drilling tool may be removed and a wireline logging tool may be deployed into the wellbore to log the formation. Logging may also be performed during other wellbore operations, such as treatment, production, etc.
The logging may be performed by sensor pads on the logging tool that collect data regarding the formations. Typically, the more surface area of the wellbore wall the sensor pads cover, the more complete the formation data collected by the sensor pads. When the drilling mud is an oil based drilling mud, the sensor pads typically send and collect high frequency current into the formation in order to determine properties of the formation. When the drilling mud is a water based drilling mud, sensor pads located proximate the wellbore may send low frequency current into the formations for collection at another location on the downhole tool.
Attempts have been made to provide downhole tools with devices such as sensor pads that retract during transport through the wellbore and then extend for contact with the wellbore wall. Some techniques may involve downhole tools with extendable arms as described, for example, in U.S. Pat. Nos. 4,614,250, 4,926,937, 4,979,585, 5,092,056, 6,702,010, 5,022,484, 7,069,775, 7,131,210. Various downhole tools with sensors positioned on extendable arms have been developed, such as the NCMI™, SHDT™, FMS™, FMI™, OBMI™, OBDT™ and HDT™ tools commercially available from SCHLUMBERGER™; the OMRI™ and EMI™ tools commercially available from HALLIBURTON™; the STAR™ and EARTHIMAGER™ commercially available from BAKER HUGHES™ INC.; and the CMI MICROIMAGER™ commercially available from WEATHERFORD™ INTL.
Despite the development of techniques involving downhole tools with extendable arms, there remains a need to provide advanced techniques for effectively positioning sensor pads against the wellbore wall. It may be desirable to provide techniques that enable logging as the downhole tool passes through the wellbore. It may be further desirable to provide techniques to enhance a contact area between multiple sensor pads and the wellbore wall. Preferably, such techniques involve one or more of the following, among others: increased contact of the sensors with the wellbore wall, increased coverage of the wellbore wall, verification of the previously logged areas, enhanced (e.g., uniform) engagement with the wellbore wall, and/or bi-directional logging of the wellbore.